Sintered body of aluminum nitride

ABSTRACT

The present invention provides a sintered body of aluminum nitride comprising (i) aluminum nitride as a main component and (ii) at least one component selected from the group consisting of Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Nd, Ho, Ti, and compounds thereof in the total amounts of not greater than 1.0 wt. % and not less than 0.01 wt. % in terms of elements on the basis of sintered body, the sintered body being colored and having a thermal conductivity of at least 150 W/mK. The sintered body is useful for the preparation of circuit boards having printed circuits thereon and highly heat-releasing ceramic packages for semiconductive devices.

This application is a continuation-in-part of application Ser. No.07/352,265 filed May 16, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to colored sintered bodies of aluminumnitride and, more specifically, to sintered bodies of aluminum nitridewhich have a high density, a high thermal conductivity and a desiredcolor, such as brownish black, brown, green, etc.

The present invention further relates to circuit boards and packagesincluding semiconductor devices in which sintered bodies of aluminumnitride above mentioned are employed. 2. Description of the Prior Art

In recent years, there has been a remarkable progress in large scaleintegration (LSI) technology and considerable improvements have beenmade in the integration techniques.

Improvements in size of integrated circuit (IC) chips contribute to suchrecent LSI innovations. As the size of IC chips has been increasinglyimproved, generation of heat for each package also becomes larger.Accordingly, the heat dissipation property of substrate materials hasbecome increasingly important. Heretofore, sintered alumina has beenemployed as an IC substrate material. However, since the thermalconductivity of the sintered alumina is not sufficient for permittingsatisfactory heat release, the conventional sintered alumina can notprovide satisfaction for such increasing heat generation of IC chips.For this, beryllia having a high thermal conductivity has been studiedas a substitute for the sintered alumina substrate, but it is difficultto handle because of its strong toxity.

Sintered aluminum nitride (AlN) has become of interest as substratematerials for circuit boards and packaging materials in thesemiconductive industry, because it inherently has a high thermalconductivity and a high electrical insulating property and is nontoxic.

As set forth above, aluminum nitride has a high thermal conductivity anda good insulating property in the state of single crystal. However, incase where sintered bodies are prepared from aluminum nitride powder bycompression-molding followed by sintering, the resultant sinteredbodies, although depending on sintering conditions, exhibit a lowrelative density i.e., at most, of the order of 70 to 80% (based ontheoretical density of 3.26 g/cm³ of aluminum nitride) and include alarge number of pores, because of poor sinterability of aluminum nitridepowder per se.

The mechanism of heat conduction of insulating ceramics, like aluminumnitride, is mainly based on transmission of phonons. However, since thedefects due to pores, impurities, etc., of the ceramics may cause phononscattering, actually there can be obtained only sintered bodies ofaluminum nitride having a low heat conduction level. Under suchcircumstances, many methods have been proposed to achieve aluminumnitride sintered products having a high thermal conductivity.

However, in order to produce aluminum nitride sintered products of highthermal conductivity, highly pure raw materials should be employed andinclusion of impurities which may be caused during the productionprocess should be held as small as possible. Sintered products ofaluminum nitride obtained in such conditions are limited to whitetransparent products or faintly colored products. There have not beenyet obtained colored sintered products of aluminum nitride having a highthermal conductivity. Accordingly, it is highly desirable to developcolored sintered bodies of aluminum nitride for applications wherelight-transmitting properties are considered as an important matter.

SUMMARY OF THE INVENTION

In view of the foregoing facts, an object of the present invention is toprovide a sintered body of aluminum nitride which has a high thermalconductivity and is colored.

Another object of the present invention is to provide substratematerials for circuit boards and packaging materials for semiconductordevices in which the sintered bodies are employed.

The present inventors have been made many studies and extensiveexperimentation to eliminate the above-mentioned disadvantages andarrived at the discovery that addition of certain elements or theircompounds to aluminum nitride is effective. The present invention hasbeen made based on this discovery.

According to one aspect (1) of the present invention, there is provideda sintered body of aluminum nitride comprising (i) aluminum nitride as amain component and (ii) at least one component selected from the groupconsisting of Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Nd, Ho, Ti, andcompounds thereof in the total proportion of not greater than 1.0 wt. %and not less than 0.01 wt. % in terms of elements on the basis ofsintered body, said sintered body being colored and having a thermalconductivity of at least 150W/mk.

In another aspect (2) of this invention, there is also provided acircuit board comprising the above-mentioned colored sintered body ofaluminum nitride comprising aluminum nitride as a main component andhaving a thermal conductivity of at least 150W/mk and printed circuitsformed onto the colored sintered body using a conductive paste.

A still further aspect (3) provides a highly heat-conducting ceramicpackage comprising a substrate made of the colored sintered body ofaluminum nitride consisting mainly of aluminum nitride and having athermal conductivity of at least 150W/mK, a semiconductor device and alead-frame.

In the present invention, as the coloring agent used to obtain thecolored sintered bodies of aluminum nitride having a high thermalconductivity, there may be mentioned the following transition elementsor their compounds.

As the additive for black coloration, there may be mentioned ZrO₂, HfO₂,V₂ O₃, Nb₂ O₃, Ta₂ O₃, Cr, Mo, WO₃, MnO, Fe₂ O₃, CoO, NiO, TiO₂, etc.Further, there may be added Nd₂ O₃, etc. for brown coloration and Ho₂O₃, etc., for green coloration. However, the coloring agents which maybe employed in the present invention are not limited to the oxideadditives as mentioned above. Any compound can be used as long as ityields the transition elements as specified above by thermaldecomposition. Examples of such compounds include carbonates,hydroxides, organic compounds of the transion metal elements.

The aforesaid coloring additives may be employed either singly or, ifdesired, in combination of two or more thereof.

Besides the foregoing additives, one or more components selected fromthe group consisting of elements of IIa and IIIa of the Periodic Tableand compounds thereof may be also used in the total proportion of 0.01to 1.0 wt. %, preferably not greater than 0.1 wt. % within the range, onthe basis of the sintered aluminum nitride body as a sintering aid.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For a better understanding, the present invention will be described inmore detail hereinafter.

In practing the present invention, aluminum nitride is used in the formof powder and preferably, has a specific surface area of at least 2.0 m²/g. When the specific surface area is less than 2.0 m² /g, sinteredbodies of high density can not be obtained.

In the production of black sintered bodies according to the presentinvention, one or more components selected from the group consisting ofthe transition elements of Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, andTi, or compounds thereof are added in the total proportion of 0.01 to1.0 wt. % based on the total weight of the resulting sintered body intoaluminum nitride powder. When the transition elements are added in acompound form, the addition amounts of 0.01 to 1.0 wt. % are given aselements throughout the present specification, unless otherwisespecified. In addition, if desired, one or more components selected fromIIa and IIIa elements of the Periodic Table and compounds thereof areadded as a sintering aid in their total of 1.0 to 0.01 wt. % on thebasis of the sintered product. When the sintering aid is used in acompound form, the addition amount is given as element. The resultingcomposition can be compacted by heating under a non-oxidizing atmosphereincluding nitrogen, thereby providing sintered bodies having a desiredcolor.

In the production of black-colored sintered bodies, highly pure aluminumnitride should be employed and the impurities should be controlled asfollows. The oxygen content and the carbon content should be limitedwithin the amounts of not greater than 2.0 wt. % and not greater than0.5 wt. %, respectively. Further, other impurities excluding transitionmetals and compounds thereof should not exceed 0.01 wt. % in terms ofelements on the basis of aluminum nitride powder. Transition metalelements and their compounds as impurities should not be exceed 0.1 wt.% in terms of elements on the basis of aluminum nitride powder in whichthe transion metals and their compounds specified above as coloringagents are excluded. If the impurities are not limited as specifiedabove, required levels of thermal conductivity and coloration can not beachieved at the same time.

In order to obtain aluminum nitride sintered bodies of high thermalconductivity which are colored in brown or green, further purifiedaluminum nitride powder should be employed. For this purpose, the purityof aluminum nitride powder is controlled so that the transition metalelements and their compounds as impurities should be limited up to 0.01wt. % in their total (calculated as elements) on the basis of aluminumnitride powder.

As the sintering agent and coloring agent to be added to such aluminumnitride powder, there are used Nd, Ho and their compounds in the rangeof 1.0 to 0.01 wt. % in their total amounts as elements on the basis ofsintered body. For example, addition of Nd will give sintered aluminumnitride colored in brown.

In the present invention, coloring agents of the same color may be usedalone or in combination of two or more thereof and coloring agents ofdifferent colors may be also used alone or in combination thereof.

The colored sintered aluminum nitride thus obtained according to thepresent invention have all a high thermal conductivity of at least150W/mK and are useful as substrates for circuit boards in which thickfilm paste (e.g. Ag paste, Au paste, etc., or a high melting-point metalpaste, for example, of W, Mo, etc.) is printed onto the surface of thedense sintered body and then fired.

Further, the substrate thus obtained are combined with a semiconductordevice and a lead-frame to assemble an IC package.

The invention will be more clearly understood with reference to thefollowing Examples. In the Examples, all percentages of the foregoingadditives to be aluminum nitride are all indicated in the term ofelements on the basis of total weight of the resulting sintered bodies,unless otherwise indicated.

EXAMPLE 1

Aluminum nitride powder [specific surface area: 3.5 m² /g (measured byBET method), oxygen content: 1.3 wt. %, and metal impurities: 0.01 wt.%] was mixed with 1.0 wt. % of phenol resin and 0.5 wt. % of Y₂ O₃ andthen additives as shown in Table were added to prepare molded bodies (30mm×30 mm×3 mm).

The molded bodies were fired at a temperature of 1950° C. for 5 hours ina stream of nitrogen gas and plate-like sintered bodies were obtained.The thus obtained sintered bodies were examined for color, thermalconductivity and density and the results are shown in Table.

It is clear from Table that the sintered bodies of aluminum nitride ofthe present invention are satisfactorily colored. Further, the sinteredbodies have been found to have a high thermal conductivity and othersuperior properties which can not be achieved in any conventionalsintered aluminum nitride.

EXAMPLE 2

A commercially available Au paste was printed onto the sintered body No.1 given in Example 1 and fired at 930° C. in air. It was found that theconductive circuit pattern formed on the sintered body had a bondingstrength of 3 kg/m². From such results, the sintered body of aluminumnitride of the present invention is useful as substrate materials forgood heat-conducting circuit boards.

                  TABLE                                                           ______________________________________                                                     Properties of sintered bodies                                                                 Thermal                                          No.  Additives     color     conductivity                                                                           Density                                 ______________________________________                                        1    Nd.sub.2 O.sub.3                                                                       0.8 wt. %                                                                              Brown   220      3.26                                  2    Ca.sub.2 CO.sub.3                                                                      0.8 wt. %                                                                              White   180      3.26                                                         transparent                                            3    TiO.sub.2                                                                              1.2 wt. %                                                                              Black   80       3.26                                  4    Ho.sub.2 O.sub.3                                                                       0.8 wt. %                                                                              Green   220      3.26                                  5    TiO.sub.2                                                                              0.8 wt. %                                                                              Black   160      3.26                                  6    ZrO.sub.2                                                                              0.8 wt. %                                                                              Black   170      3.26                                  7    V.sub.2 O.sub.3                                                                        0.8 wt. %                                                                              Black   160      3.26                                  8    WO.sub.3 0.8 wt. %                                                                              Black   180      3.26                                  9    MnO      0.8 wt. %                                                                              Black   170      3.26                                  10   Fe.sub.2 O.sub.3                                                                       0.8 wt. %                                                                              Black   160      3.26                                  11   CoO      0.8 wt. %                                                                              Black   160      3.26                                  12   NiO      0.8 wt. %                                                                              Black   170      3.26                                  13   Al.sub.2 O.sub.3                                                                       0.8 wt. %                                                                              White   100      3.26                                  14   SiO.sub.2                                                                              0.8 wt. %                                                                              White   120      3.26                                  ______________________________________                                         Note:                                                                         Nos. 2, 3, 13 and 14: Sintered bodies outside the present invention      

As described above, according to the present invention, there can beobtained sintered bodies of aluminum nitride having a high thermalconductivity. The sintered bodies of aluminum nitride are colored inblack, brown, green, etc. and transparent or semitransparent whitesintered bodies are not produced.

Accordingly, the sintered bodies of the present invention are veryuseful as excellent heat-releasing substrate materials or parts in ICsubstrates, packages or the like; in applications in whichlight-transmission is undesirable; in optical sensors which are used inautomatic operation lines, etc.

What is claimed is:
 1. A sintered body of aluminum nitride comprising(i) aluminum nitride as a main component and (ii), as a coloring agent,at least one component selected from the group consisting of Zr, V, Nb,Ta, Cr, Co, Ni, Ti, and compounds thereof in the total proportion of notmore than 1.0 wt. % and not less than 0.2 wt. % in terms of elementsbased on said sintered body, said sintered body being colored and havinga thermal conductivity of at least 150W/mK.
 2. A sintered body ofaluminum nitride as claimed in claim 1 in which said sintered bodyfurther includes, as a sintering aid, at least one component selectedfrom IIa and IIIa elements of the Periodic Table and compounds thereofin the total proportion of 1.0 to 0.01 wt. % in terms of elements on thebasis of said sintered body.
 3. A sintered body of aluminum nitride asclaimed in claim 1 in which said aluminum nitride is made from analuminum nitride powder having a specific surface area of at least 2.0m² /g measured by a BET method.